Field
Various communication systems may benefit from appropriate synchronization signal design. For example, third generation partnership project (3GPP) long term evolution advanced (LTE-A) releases 12 and 13 (Rel 12/13) may benefit from such design for device to device (D2D) communications. In particular, synchronization signals may be designed to benefit proximity services (ProSe)/D2D discovery and communication.
Description of the Related Art
One aspect of D2D operation is D2D synchronization. D2D operation may be synchronous for discovery and communication. In case of out of network (NW) coverage, user equipment (UE) based synchronization can be adopted. In case of within NW coverage, the UE can derive transmission (TX) timing based on the downlink (DL) timing and timing advance (TA), if available.
Synchronization sources, which may comprise eNB, D2D UEs, D2D cluster heads or other type of node transmitting some beacons, may transmit at least a D2D synchronization signal (D2DSS). The D2D synchronization signal may be used for time and frequency synchronization in D2D communication. This D2D synchronization signal may be used by D2D UEs at least to derive or determine time and/or frequency related information for D2D synchronization. For example, the D2D UEs may utilize the D2D synchronization signal for symbol timing acquisition, carrier frequency synchronization, and sampling clock synchronization in D2D communication. The D2D synchronization signal allows the D2D UEs to complete time and frequency synchronization and to acquire useful system parameters such as D2D device identity and/or access mode (FDD/TDD). This D2D synchronization signal may also carry the identity and/or type of the synchronization source(s). The D2D synchronization signal can include a primary D2DSS (PD2DSS), which can use a Zadoff-Chu (ZC) sequence. The D2D synchronization signal can also include a secondary D2DSS (SD2DSS), which can use a maximum length (M) sequence. The PD2DSS and SD2DSS can include, for example, an ID for a cluster head, which also could be used for scrambling of the D2D communication channel related to the cluster head (CH). The PD2DSS and the SD2DSS form part of the layer 1 device identification ID, which may be used to separate reference symbols and the content of associated channels among neighboring devices. The PD2DSS and the SD2DSS are located in different radio resources within radio frames. The PD2DSS may be used by D2D UEs to acquire or determine the slot boundary timing, for example. The PD2DSS signal or sequence may be the same for any given cell in every subframe in which it is transmitted. The SD2DSS signal or sequence may be used by D2D UEs to determine the position of the radio frame boundary.
A D2D UE searches the PD2DSS for a quick check on the presence of a D2D synchronization source. And SD2DSS may carry more extended information for D2D discovery and/or communication. A successfully detected PD2DSS and/or SD2DSS can also provide a timing and frequency reference for the D2D UE. Cellular primary synchronization signal (PSS)/secondary synchronization signal (SSS) can be used for time and frequency synchronization for communication between UEs and corresponding eNBs in a cellular network and are cell-specific reference signal. The cellular primary synchronization signal (PSS)/secondary synchronization signal (SSS), as defined in the current LTE specification, for example, may be reused for D2D synchronization. However, if the cellular PSS/SSS is reused for D2D synchronization, a UE searching for a LTE network and/or D2D transmissions may be unable to determine if the detected synchronization signal is cellular PSS/SSS or D2D PSS/SSS. In other words, the UE may not identify whether the detected PSS/SSS is coming from a D2D synchronization source for D2D communication or from an evolved Node B (eNB) for cellular communication.
If frequency division duplex (FDD) is utilized in the cellular network, the PSS/SSS from eNB may be transmitted on a DL carrier frequency while the PSS/SSS from D2D UE may be transmitted on an uplink (UL) carrier frequency assuming that D2D communication is operated or performed over UL carrier frequency. Based on the detected frequency, the UE can determine whether the PSS/SSS is for D2D synchronization or for cellular synchronization. By contrast, in case of time division duplex (TDD) operation, the same sequence may be used for both regular cellular communication and D2D operation. Thus, the receiving UE may not know whether the detected PSS/SSS is coming from an eNB for cellular communication or D2D synchronization resources for D2D communication without further checking with the transmitter of the detected PSS/SSS. The problem exists as well in FDD system if D2D is operated or performed over DL carrier frequency. And the embodiments described in the following apply to FDD case as well.